Preprints
https://doi.org/10.5194/bg-2020-59
https://doi.org/10.5194/bg-2020-59

  22 Apr 2020

22 Apr 2020

Review status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Differential effects of redox conditions on the decomposition of litter and soil organic matter

Yang Lin1,2, Ashley N. Campbell3, Amrita Bhattacharyya3,4, Nicole DiDonato5, Allison M. Thompson5, Malak M. Tfaily5,6, Peter S. Nico4, Whendee L. Silver1, and Jennifer Pett-Ridge3 Yang Lin et al.
  • 1Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720
  • 2Department of Soil and Water Sciences, University of Florida, Gainesville, Florida 32611
  • 3Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550
  • 4Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
  • 5Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
  • 6Department of Environmental Science, University of Arizona, Tucson, Arizona 85719

Abstract. Soil redox conditions exert substantial influence on biogeochemical processes in terrestrial ecosystems. Humid tropical forest soils are often characterized by fluctuating redox dynamics, yet how these dynamics affect patterns in soil versus litter decomposition and associated CO2 fluxes is not well understood. We used a 13C-labeled litter addition to explicitly follow the decomposition of litter-derived vs. native soil-derived organic matter in response to four different soil redox regimes – static oxic or anoxic, and two oscillating treatments – in soil from the Luquillo Experimental Forest, Puerto Rico. We coupled this incubation experiment with high-resolution mass spectrometry to characterize the preferential decomposition of specific classes of organic molecules. CO2 production from litter and soil organic matter (SOM) showed distinctly different responses to redox manipulation. The cumulative production of SOM-derived CO2 was positively correlated with the length of soil exposure to an oxic headspace (r = 0.89, n = 20), whereas cumulative 13C-litter-derived CO2 production was not linked to oxygen availability. The CO2 production rate from litter was highest under static anoxic conditions in the first half of the incubation period, and later dropped to the lowest among all redox treatments. In the consistently anoxic soils, we observed the depletion of more oxidized water-extractable organic matter (especially amino sugars, carbohydrates, and proteins) over time, suggesting that under anaerobic conditions, microbes preferentially used more oxidized litter-derived compounds during the early stages of decomposition. Results from kinetic modeling showed that more frequent anoxic exposure limited the decomposition of a slow-cycling C pool, but not a fast-cycling pool. Overall, our results demonstrate that substrate source – freshly added litter vs. native organic matter – plays an important role in the redox sensitivity of organic matter decomposition. In soil environments that regularly experience redox fluctuations, anaerobic heterotrophs can be surprisingly effective in degrading fresh plant litter.

Yang Lin et al.

 
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Yang Lin et al.

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Short summary
Soils in tropical forests play an important role of breaking down dead plant tissue and returning carbon to the atmosphere. This process is previously thought to be regulated soil oxygen level. However, we found that the break-down of new plant carbon by soils did not depend on oxygen. Even without oxygen, some soil microbes can use plant carbons in the forms like sugar and protein. Once these compounds were exhausted, oxygen limitation became to influence organic matter break-down.
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